During development of the central nervous system, primitive, multipotent neural stem cells (NSC) proliferate, giving rise to transiently dividing progenitor cells that eventually differentiate into the various cell types that compose the adult brain. The adult central nervous system mainly consists of neurons and glial cells, which include astrocytes and oligodendrocytes. The progenitor cells for neurons, astrocytes and oligodendrocytes originate sequentially from neural stem cells in the developing brain (see FIG. 1). Neuronal progenitor cells form first and differentiate into many types of neurons. Astrocytes develop second and function to support neuron survival. Finally, oligodendrocyte progenitor cells start to appear and migrate throughout the central nervous system. They then differentiate into mature oligodendrocytes, which produce myelin necessary for proper neuronal function.
Since oligodendrocytes play an important role in supporting the central nervous system, a pure or enriched population of oligodendrocytes or their predecessor cells (i.e., oligodendrocyte pre-progenitor cells and/or oligodendrocyte progenitor cells) would be useful for cell therapies and regenerative medicine such as in the treatment of neurological disorders including congenital demyelinating diseases (for example, Krabbe disease or Pelizaeus-Merzbacher disease), spinal cord injury and other conditions that result from defects in the myelin sheath that insulates nerve cells. These cells also can be used for research and for identifying new drugs for the treatment of many neurological disorders such as multiple sclerosis and schizophrenia.
Mature oligodendrocytes do not proliferate and do not survive well in culture, and the ability to obtain oligodendrocytes directly from tissue samples in quantities sufficient for use in research or human therapy is extremely difficult. As a result, the use of oligodendrocytes for these purposes is hindered by the lack of availability of these cells.
One solution to this problem involves obtaining neural stem cells and/or neural progenitor cells from tissue, expanding the cells in culture to obtain a sufficiently large quantity of cells which can subsequently differentiate into oligodendrocytes. Differentiation can take place either in vitro or in vivo, such as in the case of transplantation. This would result in a large population of oligodendrocytes or their progenitors or pre-progenitors for use in research and human therapy.
However, scientists have struggled to identify culture conditions that permit long term culture and mass expansion of oligodendrocyte progenitors and/or pre-progenitors—particularly from humans or non-human primates—wherein the resulting expanded cell population is primarily comprised of cells that retain the ability to differentiate into oligodendrocytes.
Several scientists have reported obtaining oligodendrocyte progenitor cells from rats ((Raff et al, J. Neurosci., 3:1289, 1983; Raff et al, Nature., 303:390, 1983; Espinosa de los Monteros et al, Proc. Natl. Acad. Sci. U.S.A., 90:50, 1993). These proliferative oligodendrocyte progenitors are known as O-2A progenitors because of their ability to differentiate in vitro into either oligodendrocytes or type 2 astrocytes. Other scientists have identified rat or mouse oligodendrocyte pre-progenitors in primary culture ((Gallo, Armstrong R C, J. Neurosci., 15:394, 1995; Grinspan, Franceschini B, J. Neurosci. Res., 41:540, 1995; Decker et al, Mol. Cell. Neurosci., 16:422, 2000). These cells are thought to be precursors of oligodendrocyte progenitors and are expected to be more beneficial in cell therapy because of their superior migration capacity as compared to oligodendrocyte progenitors. Unfortunately, scientists have been unable to effectively expand these cells for long periods of time in vitro. In contrast, scientists have reported culturing O2A progenitors from rat optic nerve or spinal cord using B104 conditioned medium or growth factor combinations such as (i) platelet derived growth factor-AA (PDGF-AA) with basic fibroblast growth factor (bFGF or basic FGF) and neurotrophin-3 (NT-3), or (ii) PDGF-AA with ciliary neurotrophic factor (CNTF) and NT-3. However, no one has succeeded in mass expansion of these cell types from primate tissue using these growth factors.
Thus, it remains very difficult to obtain and expand a pure or enriched population of oligodendrocytes and/or their predecessor cells from mammals other than rat or mouse. It is particularly difficult to obtain and expand these cells from humans and non-human primates. Therefore, a great need exists for methods for generating pure or enriched populations of mammalian neural stem cells or progenitor cells which are prone to differentiate into oligodendrocyte-lineage cells in vitro.